11.2 Loudness perception and equal-loudness contours

Loudness perception is a complex interplay between physics and psychology. It's not just about how loud a sound is, but how our brains interpret it. This topic explores the relationship between objective sound measurements and our subjective experience of volume.

Understanding loudness perception is crucial for everything from designing headphones to creating effective noise regulations. We'll dive into how our ears respond differently to various frequencies and intensities, and how scientists quantify these subjective experiences.

Loudness Perception Fundamentals

Loudness and sound pressure level

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• Loudness
  • Subjective perception of sound intensity reflects how humans experience volume
  • Psychological correlate of sound's physical amplitude varies between individuals
• Relationship to sound pressure level (SPL)
  • Measured in decibels (dB) quantifies sound pressure objectively
  • Logarithmic scale compresses wide range of pressures into manageable numbers
  • Doubling of perceived loudness ≈ 10 dB increase in SPL demonstrates non-linear relationship
• Frequency dependence
  • Human ear sensitivity varies with frequency affecting perceived loudness
  • Most sensitive range: 2-5 kHz aligns with speech frequencies
  • Less sensitive at low and high frequencies requires higher SPL for equal loudness perception

Equal-loudness contours and hearing

• Equal-loudness contours
  • Curves representing combinations of frequency and SPL perceived as equally loud illustrate hearing sensitivity
  • Measured in phons standardizes loudness across frequencies
• Fletcher-Munson curves
  • Original equal-loudness contours (1933) pioneered psychoacoustic research
  • Revised by Robinson-Dadson (1956) and ISO 226:2003 improved accuracy
• Implications for hearing sensitivity
  • Non-linear frequency response of human ear affects sound perception
  • Increased sensitivity in 2-5 kHz range enhances speech recognition
  • Reduced sensitivity at low and high frequencies impacts music and environmental sound perception
• Applications
  • Audio equipment design optimizes sound reproduction for human hearing
  • Noise control and assessment considers frequency-dependent loudness perception
  • Hearing protection standards account for equal-loudness contours

Loudness scaling in psychoacoustics

• Loudness scaling
  • Methods to quantify subjective loudness perception enable objective measurements
  • Relates physical sound parameters to perceived loudness bridges physics and psychology
• Sone scale
  • Unit of perceived loudness provides linear scale for subjective experience
  • 1 sone = loudness of 40 dB SPL at 1 kHz establishes reference point
  • Doubling of sones = doubling of perceived loudness simplifies loudness comparisons
• Stevens' Power Law
  • $L = k * I^n$ models relationship between physical intensity and perceived loudness
  • L: perceived loudness, I: sound intensity, k: constant, n: power exponent (typically 0.3 for loudness)
• Applications in psychoacoustics
  • Hearing aid design and fitting improves user experience
  • Sound quality assessment enhances product development (cars, appliances)
  • Audio compression algorithms optimize data reduction while preserving perceived quality

Loudness vs sound intensity

• Sound intensity
  • Objective measure of sound energy quantifies physical properties
  • Measured in watts per square meter (W/m²) indicates energy flow
  • Proportional to the square of sound pressure relates to measurable acoustic parameters
• Loudness vs. sound intensity
  • Loudness: subjective perception varies between individuals and contexts
  • Intensity: physical property remains constant regardless of listener
• Relationship to human perception
  • Weber-Fechner law
    • Perceived sensation is proportional to the logarithm of the stimulus intensity explains non-linear loudness perception
  • Just Noticeable Difference (JND)
    • Smallest detectable change in stimulus intensity varies with sound level
    • Varies with frequency and overall sound level affects perceptual resolution
• Factors affecting loudness perception
  • Duration of sound influences perceived loudness (temporal integration)
  • Spectral content affects perceived loudness (frequency components)
  • Temporal patterns impact loudness perception (amplitude modulation)
  • Spatial distribution of sound sources influences perceived loudness (localization cues)